Combined Influences of Water Content and Coarse Grain Content on Shear Strength of Unsaturated Soil Mixture

An interlayer existed between the ballast layer and subgrade in the conventional railway substructure. Considering that the shear strength τ of the interlayer soil was influenced by the changes in the ballast grain content and water content, this aspect was explored in the present study. Monotonic triaxial tests were fulfilled, which considered five coarse grain contents f(v) and three water contents of fine soil w(f). The results showed that the growth in f(v) contributed to an increment in τ of the soil mixture under both saturation and unsaturation. Conversely, in previous studies, the growth of f(v) induced an increment in τ under saturation, but a decline in that under unsaturation. This was explained by the competing influences of f(v) and suction ψ: in previous studies, increasing f(v) induced a decline in the dry density of the fine soil fraction ρ(d–f), which contributed to a decline in ψ. When the negative influence of declining ψ outweighed the positive influence of the incrementing f(v), the τ of the soil mixture decreased. Meanwhile, modelling of the τ–ψ relationship in the soil mixture with varying f(v) was performed. This proposed model was examined using the test results from both the present and previous studies, which shows its reasonably good performance.